Chapter 1
Introduction to Coatings

Coatings have been used since prehistoric times to protect objects and convey information, and they are ubiquitous in modern society as they serve to both protect substrates and impart aesthetic qualities to improve objects' appearance. If you are reading this text in a traditional paper book, the paper is coated. Look up and the walls of your room are coated, as are the windows. If you are wearing glasses, the lenses are likely coated to improve the plastic's scratch resistance and absorb UV radiation. If you are reading this text on a computer screen, the screen is coated to prevent glare and perhaps reduce fingerprints. The CPU inside your computer exists because of coatings used during the printing of nanometer-sized circuits. If you are outside, the buildings, cars, airplanes, roads, and bridges are all coated. Objects without coatings are less common than those with coatings!

Just because coatings science is an ancient technology does not mean that innovation has ceased. Today many coatings scientists and formulators are working diligently to improve the performance of coatings, reduce the environmental impact of their manufacture and application, and create coatings that provide functionality beyond today's coatings.

1.1 DEFINITIONS AND SCOPE

Coatings are typically thought of as thin layers that are applied to an object, which is often referred to as the substrate. Thus, one of the defining characteristics of a coating is its thinness. While the thickness of a coating depends on the purpose it serves, typical coating thicknesses range from a few microns to a few hundred microns, but of course, exceptions to this are common. Historically, the thickness of a coating was often quoted in terms of mils, where 1?mil equals one thousandth of an inch or 25.4?µm.

While coatings can be made from any material, this book is primarily concerned with organic coatings. Thus, we leave for other books coatings such as the zinc coatings used to galvanize steel, ceramic coatings that are formed from metal oxides or when metals such as aluminum are anodized, and the many other inorganic coatings used to impart hardness, scratch resistance, or corrosion protection. While these coatings are both technically and economically important, they lie mostly beyond the scope of this book.

Organic coatings are often composite materials in that they are composed of more than one distinct phase. The matrix, called the binder, holds the other components of the coating composition together and typically forms the continuous phase in the dry coating. As stated previously, we are mostly concerned with organic coatings, where the binder is typically an organic polymer.

A confusing situation results from multiple meanings of the term coating. As a noun coating is used to describe both the material (usually a liquid) that is applied to a substrate and the resultant "dry" film. As a verb, coating means the process of application. Usually, the intended meaning of the word coating can be inferred from the context. The terms paint and finish often mean the same thing as coating and also are used both as nouns and verbs. What is the difference between a coating and a paint? Not much-the terms are often used interchangeably. However, it is fairly common practice to use "coatings" as the broader term and to restrict "paints" to the familiar architectural and household coatings and sometimes to maintenance coatings for bridges and tanks. Some prefer to call sophisticated materials that are used to coat automobiles and computer components "coatings," and others call them "paints." Consumers are often familiar with the terms varnish or stain. These are types of coatings that are used to protect and beautify wood and are certainly within the scope of this book as they are typically made from polymeric binders with or without pigments.

Because we are limiting the scope of this book to organic coatings that are historically associated with paints, we are also choosing not to cover important materials such as coatings applied to paper and fabrics, decals, laminates and cosmetics, and printing inks, even though one could argue that these coatings share much in common with traditional paints. However, readers interested in those materials will find that many of the basic principles discussed in this text are applicable to such materials. Restrictions of scope are necessary if the book is to be kept to a reasonable length, but our restrictions are not entirely arbitrary. The way in which we are defining coatings is based on common usage of the term in worldwide business. For classification purposes, coatings are often divided into three categories: architectural coatings, original equipment manufacturer (OEM) coatings, and special purpose coatings.

As the coatings industry is a relatively mature industry, its growth rate typically paces that of the general economy. Like many other industries, growth has slowed in North America and Europe and has dramatically increased in Asia and South America as those economies have boomed. An estimate of the value of coatings used in each region is shown in Figure 1.1. The total value of the global coatings market was estimated to be approximately $112 billion in 2014 (American Coatings Association and Chemquest Group, 2015).

Figure 1.1 The value of coatings used in 2014.

Source: Reproduced with permission of American Coatings Association.

Figure 1.2 summarizes the estimated value and volume of coating shipments in the United States for a recent 10-year period. The effect of the economic downturn in 2008-2009 is evident (Data from American Coatings Association and Chemquest Group, 2015).

Figure 1.2 Ten-year trend in coating shipments in the United States (both gallons and dollar value).

Source: Reproduced with permission of American Coatings Association.

1.2 TYPES OF COATINGS

Architectural coatings include paints and varnishes (transparent paints) used to decorate and protect buildings, outside and inside. They also include other paints and varnishes sold for use in the home and by small businesses for application to such things as cabinets and household furniture (not those sold to furniture factories). Architectural coatings are often called trade sales paints. They are sold directly to painting contractors and do-it-yourself users through paint stores and other retail outlets. In 2014 in the United States, architectural coatings accounted for about 60% of the total volume of coatings; however, the unit value of these coatings was lower than for the other categories, so they made up about 49% of the total value. This market is the least cyclical of the three categories. While the annual amount of new construction drops during recessions, the resulting decrease in paint requirements tends to be offset by increased repainting of older housing, furniture, and so forth during at least mild recessions. Latex-based coatings make up about 77% of architectural coatings. Interior paints are approximately 2/3 of all architectural coatings, exterior paints 23%, and stains 7%, with the remained split among varnishes, clear coats, and others.

OEM coatings are applied in factories on products such as automobiles, appliances, magnet wire, aircraft, furniture, metal cans, and chewing gum wrappers-the list is almost endless. In 2014 in the United States, product coatings were about 29% of the volume and 31% of the value of all coatings. The volume of product coatings depends directly on the level of manufacturing activity. This category of the business is cyclical, varying with OEM cycles. Often, product coatings are custom designed for a particular customer's manufacturing conditions and performance requirements. The number of different types of products in this category is much larger than in the others; research and development (R&D) requirements are also high.

Special purpose coatings are industrial coatings that are applied outside a factory, along with a few miscellaneous coatings, such as coatings packed in aerosol containers. This category includes refinish coatings for cars and trucks that are applied outside the OEM factory (usually in body repair shops), marine coatings for ships (they are too big to fit into a factory), and striping on highways and parking lots. It also includes maintenance paints for steel bridges, storage tanks, chemical factories, and so forth. In 2012 in the United States, special purpose coatings made up about 11% of the total volume and 20% of the total value of all coatings, making them the most valuable class. Many of today's special purpose coatings are the product of sophisticated R&D, and investment in further improvements remains substantial.

Coatings are used for one or more of three reasons: (1) for decoration, (2) for protection, and/or (3) for some functional purpose. The low gloss paint on the ceiling of a room not only fills a decorative need but also has a function. It reflects and diffuses light to help provide even illumination. The coating on the outside of an automobile adds beauty to a car and also helps protect it from rusting. The coating on the inside of a beverage can have little or no decorative value, but it protects the beverage from the can. (Contact with metal affects flavor.) In some cases, the interior...